Illustration: Fig. 7.—Yellow Elastic Tissue. (Highly magnified.) 22. Areolar or Cellular Tissue. This consists of bundles of delicate fibers interlacing and crossing one another, forming irregular spaces or meshes. These little spaces, in health, are filled with fluid that has oozed out of the blood-vessels. The areolar tissue forms a protective covering for the tissues of delicate and important organs. 23. Adipose or Fatty Tissue. In almost every part of the body the ordinary areolar tissue contains a variable quantity of adipose or fatty tissue. Examined by the microscope, the fat cells consist of a number of minute sacs of exceedingly delicate, structureless membrane filled with oil. This is liquid in life, but becomes solidified after death. This tissue is plentiful beneath the skin, in the abdominal cavity, on the surface of the heart, around the kidneys, in the marrow of bones, and elsewhere. Fat serves as a soft packing material. Being a poor conductor, it retains the heat, and furnishes a store rich in carbon and hydrogen for use in the body. 24. Adenoid or Retiform Tissue. This is a variety of connective tissue found in the tonsils, spleen, lymphatic glands, and allied structures. It consists of a very fine network of cells of various sizes. The tissue combining them is known as adenoid or gland-like tissue. Illustration: Fig. 8.—Fibro-Cartilage Fibers. (Showing network surrounded cartilage cells.) 25. Cartilage. Cartilage, or gristle, is a tough but highly elastic substance. Under the microscope cartilage is seen to consist of a matrix, or base, in which nucleated cells abound, either singly or in groups. It has sometimes a fine ground-glass appearance, when the cartilage is spoken of as hyaline. In other cases the matrix is almost replaced by white fibrous tissue. This is called white fibro-cartilage, and is found where great strength and a certain amount of rigidity are required. Again, there is between the cells a meshwork of yellow elastic fibers, and this is called yellow fibro-cartilage (Fig. 8). The hyaline cartilage forms the early state of most of the bones, and is also a permanent coating for the articular ends of long bones. The white fibro-cartilage is found in the disks between the bodies of the vertebræ, in the interior of the knee joint, in the wrist and other joints, filling the cavities of the bones, in socket joints, and in the grooves for tendons. The yellow fibro-cartilage forms the expanded part of the ear, the epiglottis, and other parts of the larynx.